1. Field of the Invention
The present invention relates generally to impact printing devices for dot matrix printing wherein at least one print wire is propelled against a printing medium by an associated plunger type solenoid print wire driver for printing dot matrix characters in accordance with external control signals which cause plunger coil energization and consequent character printing. More particularly, the present invention relates to an improved print wire solenoid driver having a rapid cycle repeat time and a low loss magnetic circuit of high efficiency and durability.
2. Description of the Prior Art
Modern high speed matrix printers must have print heads capable of printing a variety of fonts at ever increasing speeds, while maintaining reliability of operation, cost efficiency and durability. Prior art print heads having plunger type solenoids for driving print wires generally suffer from the inability to achieve low enough plunger cycle repeat times to enable the printer to operate at peak printing speeds. This deficiency of prior art solenoid print wire drivers results from a variety of factors; among which are: excessive plunger mass which reduces plunger acceleration, non-working air-gaps in the solenoid magnetic circuit, which increase reluctance in the magnetic circuit with a consequent decrease in efficiency. Other problems associated with prior art solenoids include, less than optimum flux density in the plunger, and electrical heating in the plunger coil, both of which reduce the solenoid efficiency.
One such prior art solenoid for a wire printer is described by U.S. Pat. No. 3,787,791, which prior art solenoid is more completely described with reference to FIG. 1 herein. As will be described, this prior art solenoid contains two air gaps, one of which is non-working and the other of which, while developing a maximum force for a given magnetic flux, requires a maximum magnetomotive force derived from additional ampere-turns of coil to develop that force, adding to the electrical resistance of the plunger coil and decreasing the solenoid efficiency.